Floating structures, such as drill ships and semisubmersibles, have long been used to drill and complete subsea wells. Most of these floating structures have a large-diameter cylindrical opening, known as a moonpool, which extends vertically through the hull of the structure. The moonpool provides access from a work platform on the upper end of the structure into the water below. Most drilling and completion operations are carried out through the moonpool.
The work platform on a floating structure must remain relatively stable during the drilling and completion of a sebsea well. In extremely deep waters or in areas subject to high waves and strong winds, providing a stable floating platform can be a problem. One type of floating structure which is particularly adapted for use in such environments is the cassion vessel. A typical caisson vessel has an elongated, cylindrically-shaped hull, formed of concrete. The vessel has ballast tanks or compartments on its lower end and a work platform at its upper end. When in an operable position, the hull is moored vertically in the water with a substantial portion of the hull being submerged.
The submerged portion of the hull of the caisson vessel provides stability for the work platform. Once submerged, the bottom of the vessel may be as much as 500 feet below the surface of the water. Thus the moonpool, extending from the work platform completely through the hull, may also be 500 feet long. Unfortunately, it is difficult to raise or to lower drill strings and other such equipment through long moonpools without damaging the moonpool walls or the equipment being handled. Therefore, in moonpools of this type, some device must be provided which will engage the drill string or riser, and center and guide that drill string or riser while it is being lowered or raised through the moonpool.
If only the tubular members of a typical drill string or riser were involved, permanent guides could be positioned at desired intervals throughout the length of the moonpool. Sometimes, however, large equipment packages (such as BOP stacks and wellhead assemblies) must be attached to a drill string and run through the moonpool. Accordingly, for a moonpool guidance system to meet the practical requirements of routine drilling and completion operations, it must designed so that it may be moved out of the way to allow the unobstructed passage of such equipment packages. This retraction to provide unobstructed passage should be accomplished without the necessity of withdrawing the guidance system components from the moonpool.
Finally, due to the construction and flexibility of the different drill strings involved in the operations routinely carried out through the moonpool, the guidance system must be capable of moving to different depths within the moonpool. This allows the system to provide guidance for any particular drill string at the points most appropriate for that string. It also allows the guidance apparatus to maintain its position relative to a drill string while that string is being raised or lowered through the moonpool.